1. Field of the Invention
This invention relates to an ignition control system, and more particularly to an ignition control system for protecting an internal combustion engine from overspeed.
2. Description of Related Art
FIG. 10 shows ignition control timings of a conventional internal combustion engine. When a driver turns on an ignition at the start of the internal combustion engine (hereinafter referred to as xe2x80x9cenginexe2x80x9d), current is supplied to a control unit for controlling the engine and a variety of sensors and actuators attached to the engine. If the driver then turns an ignition switch, a starter is directly connected with a battery to crank the engine. If the engine is cranked, the control unit controls the engine so as to start injecting fuel to each cylinder and ignite fuel in each cylinder. It is known that the supply of current to an ignition coil is stopped for extinguishment in order to protect the engine from overspeed. In the case of a transistor ignition coil control system, it is necessary to stop the supply of current in order to extinguish the fuel in the cylinder. If the overspeeding state is detected in a timing 2-1 in FIG. 10, however, a cylinder 2 that is currently supplied with current is ignited even if the supply of current is stopped, because the supply of current starts at a point P1 and the current-carrying period required for ignition has already elapsed. The supply of current to subsequent cylinders is canceled for extinguishment. For this reason, the overspeed protection may be delayed.
Since the overspeeding state is very dangerous to internal combustion engines, it is preferable to reduce the time from the detection of the overspeeding state to the start of the overspeed protection. According to the above-mentioned prior art, the overspeed protection is started from a cylinder that is supplied with current next time, and thus, the ignition control system cannot achieve the satisfactory responsiveness.
It is therefore an object of the present invention to provide an ignition control system that improves the responsiveness to overspeed by reducing the time period from the detection of the overspeeding state to the start of the overspeed protecting operation without deteriorating the normal ignition control characteristics.
In view of the above object, an ignition control system according to the present invention comprises: engine speed detecting means for calculating an engine speed according to an engine crank angle sensor signal; current supply start control means for outputting a proper current supply start timing signal corresponding to the engine speed to an ignition coil; current supply finish control means for outputting a proper current supply finish timing signal corresponding to the engine speed to the ignition coil; ignition coil control means for controlling supply of current to the ignition coil according to the signals outputted from the current supply start control means and the current supply finish control means; and overspeed determination means for outputting an overspeed determination signal when determining that the engine speed calculated by the engine speed detecting means is in excess of a predetermined limit value, wherein the current supply finish control means resets a timing for finishing the supply of current to the ignition coil being supplied with current to a timing on and after a time indicated by the current supply finish timing signal to thereby inhibit engine from revolving.
The overspeed determination signal may prohibit the current supply start control means from starting the supply of current, and may cause the current supply finish control means to retard the current supply finish timing so as to decrease engine torque.
Also, the overspeed determination signal may cause the current supply finish control means to retard the current supply finish timing so as to decrease engine torque.
The overspeed determination signal may prohibit the current supply finish control means from finishing the supply of current to thereby enable extinguishment.
The ignition control system according to the present invention may further comprise power supply control means for controlling supply of power to the ignition coil control means according to the overspeed determination signal. In this case, the overspeed determination signal causes the power supply control means to stop supplying power to the ignition coil control means, and prohibits the current supply finish control means from finishing the supply of current to gradually discharge a voltage having been supplied to the ignition coil control means to thereby enable extinguishment of the ignition coil.